Avoid duplicate group-, meeting- and committee-names in their context

[r-peschke]

Context means, that

• group names have to be unique in their meeting
• meetings in their committee
• and committees in the organization

We have to specify this in more detail looking for all opportunities to create or activate such objects again

[jsangmeister]

Meeting names should not become unique since there may be multiple meeting with the same name, but different dates, e.g. for recurring events.

Groups and committees should not have duplicates and can therefore just be enumerated if duplicates occur.

We should wait with this until the remove-template-fields branch is merged.